Preparation of polymers that are formed by curing or crosslinking a resin or prepolymer, e.g., the curing of an epoxy resin or polyurethane prepolymer, is conceptually simple and in many cases straightforward. However, when preparing high performance materials and articles, such as high performance polyurethane elastomers, balancing the need for quick and efficient curing with the time needed to properly process the prepolymer/curative composition before curing occurs can be problematic. Often, factors that provide a stable, easily handled and stored material tend to diminish the reactability of the system leading to longer cure times. Balancing these needs can become more difficult when there is a need to store and/or handle the reactive prepolymer and curative together for extended periods of time without causing unwanted reaction, degradation or loss of activity.
Curing or crosslinking polymer resins and prepolymers, such as epoxy resins, isocyanate-terminated polyurethane prepolymers and the like, with di-amines, tri-amines and other polyamines is well known. For example, a polyfunctional amine, e.g., bis orthochloroaniline (MOCA), methylene bis diethylaniline (MDEA), 4,4′-methylenedianiline (MDA), etc., can be mixed with a resin or prepolymer, the mixture can then be cast into a mold or onto a surface etc., and heated to complete the curing reaction. In some applications problems have arisen due to premature reaction of the curing agent with the resin or prepolymer during mixing, casting, or other processing step.
U.S. Pat. Nos. 3,755,261, 3,876,604 and 4,282,344 disclose the cure of amine curable polymers using as the curing agent co-ordination complexes formed between polyamines and alkali metal salts rather than the free amine. For example, the coordination complex of NaCl and 4,4′-methylenedianiline rather than free 4,4′-methylenedianiline can be added to an isocyanate capped prepolymer to affect cure. It is usually necessary to liberate or de-block the free amine, e.g., 4,4′-methylenedianiline, from the coordination complex, typically by heating, before it can effectively cure the resin or prepolymer.
The most commonly encountered coordination complex curing agents are alkali metal salt complexes with MDA, also called MDA-CC, for example, a 3:1 NaCl:MDA complex. Isocyanate capped prepolymers have been cured with this curative, or a similar blocked curative, to give high performance elastomeric materials.
Mixing MDA-CC with polyurethane prepolymers at temperatures below the deblocking temperature form curing compositions that often have long pot lives, which can be up to a year or more depending on the system and storage conditions. Heating the curing compositions above the decomposition temperature of the MDA-CC releases the highly active MDA, which reacts with the prepolymer to form elastomeric products. This method has been successful in many instances, including commercial ‘one pack’ polyurethane systems, but improvements are still needed. For example, in some cases deblocking and curing is sluggish, requiring the use of higher than desirable temperatures or overly long cure times. As a result, catalysts or cure accelerators are used in some applications to lower the MDA-CC deblocking temperature, allowing processors to use lower molding temperatures and increase productivity by decreasing the molding cycle time.
Added difficulties can arise in some applications, such as in the production of thicker films or articles, where non-uniform curing can occur due to rapid MDA-CC de-blocking and prepolymer curing at the surface of the heated mold, while the interior section of the composition cures more slowly or incompletely. This can result in a polyurethane elastomer lacking a consistent or uniform structure. For example, a hard skin may first form on the outer surfaces and, as the cure proceeds, the skin may rupture, resulting in an undesirable cracked surface.
Cure accelerators are catalytic compounds useful in accelerating the deblocking of blocked curatives, typically by lowering the temperatures needed for deblocking. Many such materials, e.g., glycerol and urea, have been developed for use in curing polyurethanes. However, the performance demonstrated by known cure accelerators leaves much room for improvement, and many of these catalysts may work only with certain prepolymer/curative combinations.
U.S. Pat. No. 3,888,831 discloses the curing of amine-curable polymers or prepolymers with a salt complex of methylenedianiline in association with a polar or polarizable compound, e.g., an ester, a ketone, an ether, an aromatic hydrocarbon, a halogenated hydrocarbon, a tertiary amine, a sulfone, a sulfoxide, or a sulfide. Acetone and esters of tetra-ethylene glycol are preferred. Nitrobenzene, xylene, chlorobenzene, tetra-ethylene glycol bis-2-ethyl hexanoate and dipropylene glycol dibenzoate are exemplified.
U.S. Pat. No. 4,046,743 discloses the use of a MDA complex as a curing agent of amine curable polymers or prepolymers in the presence of a high dielectric constant catalytic compound such as tributyl phosphate.
U.S. Pat. No. 4,517,331 discloses curing a polypropylene ether glycol prepolymer using a complex of MDA in the presence of a pseudocrown ether. U.S. Pat. No. 4,772,676 discloses the use of alcohol catalysts, such as, 1,4-butane diol or phenoxypoly(oxyethylene) ethanol, to effect ambient temperature cure of urethane prepolymers or polymers with a methylenedianiline (MDA) salt complex.
U.S. Pat. Nos. 8,754,184 and 9,006,375 disclose accelerating the cure rate of a polyurethane prepolymer with a methylenedianiline metal salt coordination complex by adding a nitrogen-containing organic salt, e.g., an imidazolium, pyridinium, pyrrolidinium, piperidinium, or morpholinium salt, to the prepolymer before reaction with the methylenedianiline metal salt coordination complex chain extender.
Despite the generic disclosure of such a wide range of compounds as cure accelerators, many of these compounds, or classes of compounds, have never been exemplified in a polyurethane curing composition with a polyamine coordination complex. In many cases, little is known about the impact the presence of these compounds may have on storage stability of the prepolymer or curative, and in some cases the disclosed cure accelerators are known to be detrimental to the stability of the prepolymer.
The ideal cure accelerator should 1) be compatible with the prepolymer, curing agent, and a mixture of prepolymer and curing agent, 2) would not initiate any reaction until the final curing composition is heated to the activation temperature, and then 3) at that temperature cure would be quick and consistent throughout the composition. Many known cure accelerators fall short of these criteria. For example, recent tests run by the present inventor, and reported herein, show that some of the disclosed accelerators are relatively ineffective under certain conditions and many have a negative impact on the storage stability of the prepolymer.
Thus the need exists for improved accelerators for use in the formation of polyurethanes by reaction of prepolymers with blocked curatives, especially for use with coordination complexes of metal salts and polyamines, in order to increase productivity and to produce high performance polyurethane elastomers with substantially uniform consistency.
It has been surprisingly found that alkyl sulfoxides, such as DMSO, butyl sulfoxide, ethyl butyl sulfoxide etc., not only form stable compositions with prepolymers that can be stored without degradation or loss of activity, but the presence of the sulfoxides also have exceptionally high catalytic activity relative to other disclosed cure accelerators in deblocking of MDA co-ordination complexes, and the curing of polyurethane prepolymers. Whereas prepolymer stability and catalytic activity are typically inversely related, it has been found that the inclusion of a small amount of alkyl sulfoxide, e.g., dimethyl sulfoxide, in an isocyanate capped polyurethane prepolymer composition, results in an exceptionally stable composition with unexpectedly high curing efficiency when cured with a polyamine coordination complex such as a co-ordination complex of NaCl and 4,4′-methylenedianiline.